In order to perform long-term monitoring of underground formations, such as reservoirs in the oil and gas industry, it has been proposed to install sensors in boreholes drilled in and around the formations of interest. A typical system for drilling the boreholes comprises a hollow drill string formed from pipes connected end-to-end, and a drill bit positioned on the lower end of the drill string. The presence of the drill bit has meant that it is not possible to easily obtain access to the borehole for sensors from the inside of the drill string. It is usually necessary to completely withdraw the drill string from the borehole before any sensors can be installed. U.S. Pat. No. 5,206,840 describes one method for implanting geophones (seismic sensors) in wells using a pipe or drill string. In one method, the drill string is withdrawn from the well and an open-ended drill pipe is introduced into the borehole, through which the sensors can be installed. In another method, the drill bit is sheared from the end of the drill string and the sensors pumped into the borehole. While this avoids the need to withdraw the drill string, it does mean the loss of the drill bit for each borehole.
It has recently been proposed to conduct operations below a drill bit by using a special bit with a passageway allowing access to the borehole from the inside of the drill string. One type of such operation allows interaction with the borehole ahead of the bit, for example to introduce fluids, or drill pilot holes or the like. Examples of these techniques can be found in WO 03/008754, US 2004/0238218 and US 2004/0238224. Another type of similar operation is through bit logging as is described in WO 00/17488.
All of these known techniques suffer from problems if used to install permanent sensors. The system of U.S. Pat. No. 5,206,840 requires either the complete removal of the drill string prior to introduction of open-ended drill pipe, or shearing of the drill bit.
The though bit techniques described in WO 03/008754, US 2004/0238218, US 2004/0238224 and WO 00/17488 are all based on procedures in which the drill string remains in the borehole for further drilling operations, and for which all equipment is withdrawn from the well on completion of the activity.
It is an object of the invention to provide a method for installing sensors which does not suffer from these problems.